1. Field of the Invention
The invention is related to the field of electromagnetic well logging instruments and methods. More specifically, the invention is related to an apparatus and method for reducing the effect of eddy currents induced in a permanent magnet upon the measurements made by nuclear magnetic resonance ("NMR") well logging instruments.
2. Description of the Related Art
Electromagnetic well logging instruments include circuits connected to antennas which induce alternating electromagnetic fields in earth formations surrounding a wellbores, and include circuits which measure various electromagnetic phenomena which occur as a result of interaction of the alternating electromagnetic fields with the earth formations. Such electromagnetic phenomena relate to petrophysical properties of interest of the earth formations. One type of electromagnetic well logging instrument which suffers deleterious effects of eddy currents in electrically conductive elements of the logging instrument is the nuclear magnetic resonance ("NMR") instrument. One type of NMR instrument is described in U.S. Pat. No. 4,710,713 to Taicher et al. Another type of NMR instrument is described in U.S. Pat. No. 4,350,955 to Jackson et al. Both the Taicher et al '713 instrument and the Jackson et al '955 instrument include permanent magnets for inducing a static magnetic field in earth formations, and an antenna through which pulses of radio frequency ("RF") energy are conducted. RF energy conducted through the antenna induces an RF magnetic field in the wellbore, in any electrically conductive elements of the NMR instrument and in the earth formations surrounding the instrument. The RF energy passing through the antenna of the NMR instrument therefore causes eddy currents to flow in the wellbore, in the earth formation surrounding the NMR instrument and in any electrically conductive elements in the NMR tool.
In the Jackson et al '955 patent the antenna acts as a three-dimensional dipole. The direction of a magnetic field generated by the antenna is generally along the direction of the dipole and parallel to its longitudinal axis. This type of antenna is generally referred to as a longitudinal dipole. The antenna induces an RF magnetic field in the wellbore, in the earth formations surrounding the tool and in the permanent magnet material on both sides of the dipole along the longitudinal axis of the tool. To induce an RF magnetic field in the earth formations having sufficient amplitude to make useful NMR measurements, the antenna must also necessarily generate a relatively strong RF magnetic field within the permanent magnet. If the permanent magnet material is electrically conductive, losses of RF power will occur as a result.
The apparatus disclosed in the Taicher et al '713 patent includes a substantially cylindrical permanent magnet assembly which is magnetized perpendicular to its longitudinal axis. This magnet can be modeled as an infinitely long two-dimensional dipole. The magnet induces a static magnetic field in the wellbore and in the earth formations which has substantially uniform magnetic field strength within any thin annular cylindrical volume at a predetermined radial distance from the magnet. The Taicher et al '713 apparatus also includes an antenna, wound around the exterior of the magnet, for generating the RF magnetic field and for receiving NMR signals. This antenna can be modeled as an infinitely long two-dimensional dipole. The direction of the magnetic field generated by this antenna is generally perpendicular to its longitudinal axis. This type of antenna is referred to as a transversal dipole antenna. The permanent magnet's dipole is coaxial with and orthogonal to the RF magnetic dipole.
The apparatus disclosed in the Taicher et al '713 patent has several drawbacks. In particular, the antenna induces an RF magnetic field in the formations surrounding the tool which decreases in strength as the square of the radial distance from the magnet axis. Therefore, to induce an RF magnetic field in the earth formations having sufficient amplitude to make useful NMR measurements within a sensitive volume in the earth formations, the antenna must generate a very strong RF magnetic field, which is also very strong within the space that is occupied by the permanent magnet. If the magnet is made from electrically conductive permanent magnet material, significant losses of RF power will occur as a result of eddy currents flowing in the magnet. The apparatus disclosed in the Taicher et al '713 patent is generally useful only with an electrically non-conducting permanent magnet material such as ferrite.
Another NMR logging instrument is described in U.S. Pat. No. 5,055,787 to Kleinberg et al. This logging instrument includes permanent magnets arranged to induce a magnetic field in the earth formation having substantially zero field gradient within a predetermined sensitive volume. The magnets are arranged in a portion of the tool housing which is typically placed in contact with the wall of the wellbore. The antenna in the instrument described in the Kleinberg et al '787 patent is positioned in a recess located external to the tool housing, enabling the tool housing to be constructed of high strength material such as steel. This outside metallic structure also serves as a shield against RF alternating electromagnetic fields penetrating into the permanent magnet and resulting in RF power losses in the magnet.
Although instrument in the Kleinberg et al '787 patent reduces eddy current losses in electrically conductive elements of the tool by shielding the permanent magnet, this concept has several significant drawbacks. One such drawback is that the instrument's sensitive volume is only about 0.8 cm away from the tool surface and extends only to about 2.5 cm radially outward from the tool surface. Measurements made by this instrument tool are therefore subject to large error caused by roughness in the wall of the wellbore, deposits of the solid phase of the drilling mud (called "mudcake") onto the wall of the wellbore in any substantial thickness, and by the fluid content of the formation in the invaded zone.
Another way to reduce eddy current losses in the permanent magnet in an NMR logging apparatus is described in U.S. Pat. No. 5,376,884 and No. 5,486,761 to Sezginer. The instruments described in these patents use side-by-side spaced apart elongated magnets and an RF loop in the region between the magnets. Such an arrangement enables using relatively powerful permanent magnets, such as rare-earth magnets, provided that the permanent magnets are properly shielded. The basic disadvantage of the approach taken in the '884 and '761 patents is that the relatively large conducting surfaces will disturb the spatial distribution of the RF magnetic field while transmitting, and will reduce the signal to noise ratio ("S/N") while receiving NMR signals.
Finally, the signal to noise ratio of electromagnetic well logging instruments, particularly NMR tools, is generally greater at higher values of quality factor (Q) of the instrument's antenna. The relationship of the S/N with respect to the Q of the antenna is a primarily a matter of the particular instrument geometry. High magnitudes of eddy current loss restricts the Q of the antenna, thereby restricting the useful geometry of the logging instrument.
Generally speaking, the measurement approaches suggested in the Jackson et al '950 and the Taicher et al '713 patents are commercially preferred for making NMR measurements of earth formations. However, the apparatus described in both of these patents are preferably used with substantially non-conductive permanent magnets. Magnetic materials used to make permanent magnets generally fall into two classes: ferrites, which are oxides of ferromagnetic metals; and ferromagnetic metals and their alloys combined with other metals and/or rare earth elements. The first class generally consists of non-conductive permanent magnet materials, and the second class generally consists of electrically conductive materials. Both classes of permanent magnet materials can be used in making so-called "bonded" permanent magnets. Bonded permanent magnets are generally manufactured by pressure bonding or injection molding of magnet material powders in a carrier matrix. The carrier matrix is typically formed from an electrically non-conductive polymeric or epoxy resin. The magnet material density of this form of magnets is lower than magnets made entirely from sintered metallic materials, yielding lower magnetic strength properties in the final product. However, bonding or injection molding of permanent magnets often makes it possible to eliminate the need for costly secondary operations in the manufacturing process. See for example, "New Resin-Bonded Sm-Co Magnet Having High Energy Product (SAM)", Proceedings of the Fourth International Workshop on Rare Earth-Cobalt Permanent Magnets and Their Applications, Hakone National Park, Japan (1979).
The electrical resistivity of any particular bonded permanent magnet depends primarily on the resistivity of the magnet material powder, the proportion of the magnet material powder relative to the proportion of the carrier matrix in the finished magnet, and the particular manufacturing method which determines the degree of contact between the individual grains of the magnet material powders. While they are electrically non-conductive, ferrite magnet materials have low residual magnetization, generally about three times weaker than other magnet materials such as rare-earth Samarium-Cobalt, Alnico or Neodymium-Iron-Boron, all of which are very good electrical conductors. Ferrites are also about one and a half times weaker than some commercially available bonded rare-earth Samarium Cobalt or Neodymium-Iron-Boron magnets, which are considerably less conductive than sintered magnets made from the same materials. However, the particle size of the magnet material powders and material proportion, as well as the bonding process, generate significant eddy current losses within the conductive particles themselves and between them in the overall magnet structure.
Other disadvantages of using ferrite magnet material are that the ceramic-like material is very brittle and tends to chip. This feature is particularly undesirable in the well logging environment, where logging instruments are placed under enormous hydrostatic pressure and are subjected to severe mechanical shock. Ferrites also have low coercive force (Hc) which may lead to irreversible demagnetization, and ferrite loses its magnetization at a rate of about 0.2% per degree C. Temperatures in some wellbores can exceed surface temperatures by 100 to 150 degrees C. These temperatures reduce the permanent magnet material's residual magnetization (Br) by about 20% to 30% and can reduce the magnet material's coercive force (Hc) substantially, thereby leading to irreversible demagnetization of a permanent magnet made from ferrite.
Certain magnet materials have particular disadvantages when used in well logging applications. NMR magnetoacoustic ringing is known to be a particular concern due to high acoustic quality factor of ceramics. Alnico magnet materials have very low coercive force (Hc) which may lead to irreversible demagnetization. Both Alnico and Neodymium-Iron-Boron magnets have very poor temperature characteristics and therefore are not preferred for well logging tools.
Using resin-bonded Samarium-Cobalt or resin bonded Neodymium-Iron-Boron magnets is known in the art. For example, British patent application no. 2,141,236 filed by Clow et al on May 23, 1984 shows an NMR well logging instrument similar in configuration to the instrument shown in the Jackson et al '955 patent described earlier. The Clow et al '236 application states that the instrument uses resin-bonded Samarium-Cobalt magnets. The Clow et al '236 application does not describe any particular limitations on the structure or composition of the magnet material used in the instrument, and as a practical matter, the instrument described by Clow et al has never become commercially accepted primarily because excessive conductivity of the Samarium-Cobalt magnets distorts the RF magnetic field generated by the antenna. Using resin-bonded Samarium-Cobalt or resin bonded Neodymium-Iron-Boron magnets made according to processes known in the art has generally not proven suitable in commercially preferred types of NMR well logging apparatus, such as described in the Taicher et al '713 patent, because of the electrical conductivity of such magnets.